The demand for President Bashar Assad's ouster is echoed by the governments of Canada and the European Union. The U.S. also expands its sanctions. The pressure may have a limited effect, however.

Reporting from Washington and Beirut — After months of criticism that he was too soft on Syria's government, President Obama led a choreographed call by Western governments for President Bashar Assad to give up power, a move that leaves the autocratic regime more isolated, though not necessarily less dangerous.

The demand for Assad's ouster, echoed by the governments of France, Britain, Germany, Canada and the European Union, followed weeks of diplomatic talks aimed at presenting a common front against Assad's government. The Obama administration had hoped that Turkey and Saudi Arabia, two regional powers that hold greater influence over Syria, would join the coordinated diplomatic press.

Those countries neither supported nor condemned Obama's call for Assad to go, a silence widely interpreted as an expression of frustration over the Syrian leader's unfulfilled promises to end the violence.

On Thursday, Assad's government showed no sign of ceasing its attacks on protesters, who have steadily built a widespread movement in the teeth of the violence.

Until now, the administration had suggested that Assad still held enough legitimacy to preside over reforms that would loosen his autocratic grip. But with the killing and mass arrests showing no signs of abating, Obama said Thursday for the first time that Assad was unfit to lead the country.

Assad's pledges of reform have "rung hollow while he is imprisoning, torturing, and slaughtering his own people," Obama said in a statement.

"For the sake of the Syrian people, the time has come for President Assad to step aside."

European governments made the call a chorus and said they were open to further EU sanctions on Syria.

But the Europeans, who buy 90% of Syria's oil exports, did not announce any decision to halt those imports.

That reluctance highlighted the limited leverage Washington has been able to bring to assist a widening popular revolt that has seen an entrenched regime deploy combat troops, tanks and warships against unarmed civilians, killing thousands of people.

Washington officials have ruled out any U.S. military action.

The Obama administration recently imposed several rounds of sanctions targeting Assad and other top officials. Obama expanded those efforts Thursday with an executive order that freezes all Syrian government assets in U.S. jurisdictions and bars Americans from any business with the regime, including trading in Syrian oil and selling it refined petroleum products.

But administration officials pointed out that Washington has kept sanctions on Syria for decades with little effect, and there is little American trade with Syria to stop. As a result, officials made it clear that Obama's announcement may have a limited effect, and Assad's downfall is far from assured.

"Nothing about this is going to be easy," a senior administration official, who spoke on condition of anonymity, told reporters in a telephone briefing.

Administration officials said they still are urging Saudi Arabia, other Arab governments and Turkey to take a greater role in helping steer Syria toward a new order.

This week, Secretary of State Hillary Rodham Clinton appeared to play down the importance of a U.S. call for Assad's departure. But, she added, "if Turkey says it, if [Saudi] King Abdullah says it, if other people say it, there is no way the Assad regime can ignore it."

It was unclear how forcefully those governments would act, however.

The Turkish government, Syria's biggest trading partner, is ambivalent about the crisis, in part because of its economic interests. Although Prime Minister Recep Tayyip Erdogan and other leaders have harshly condemned Assad, they have not moved as aggressively as the Obama administration would like.

Saudi Arabia, which has powerful influence with the Sunni Muslim majority of the Alawite-led country, also has condemned Assad and recently withdrew its ambassador. It did not react immediately to the U.S. and European actions.

The leaders of Britain, France and Germany joined Obama in demanding Assad's resignation.

"We call on him to face the reality of the complete rejection of his regime by the Syrian people and to step aside in the best interest of Syria and the unity of its people," said a joint statement that was signed by British Prime Minister David Cameron, French President Nicolas Sarkozy and German Chancellor Angela Merkel.

The Syrian government said the call for the regime's removal would lead to more bloodshed.

"It is strange that instead of offering [Damascus] a helping hand to implement its program of reforms, the West and Obama are seeking to stoke more violence in Syria," Reem Haddad, a government spokeswoman, told Agence France-Presse.

The Syrian government insists nameless armed Islamic extremists are causing much of the violence in the country. United Nations Secretary-General Ban Ki-moon said Assad had promised him on Wednesday that his forces would cease offensive operations Thursday.